1. Field of the Invention
The present invention relates to a carriage driving apparatus for driving a carriage by using a voice coil type DC motor.
2. Description of the Related Art
An information recording/reproducing apparatus for recording and reproducing information by using a recording medium, such as an optical card, includes a carriage designed to reciprocate with an optical card mounted thereon. An optical head is arranged to oppose the optical card mounted on this carriage. The optical head is mounted on another carriage. This carriage is designed to reciprocate in directions perpendicular to the moving directions of the optical card.
FIG. 1 shows a conventional carriage driving apparatus for driving a carriage 21 on which an optical card 20 is mounted. This carriage driving apparatus comprises a voice coil type linear DC motor 30, a pair of guide shafts 22a and 22b arranged on both sides of the DC motor 30, and a carriage 21 supported between these guide shafts 22a and 22b and fixed on a movable coil (not shown).
The voice coil type linear DC motor 30 has side yokes 31a and 31b and a center pole 32. The two end portions of each of these members are coupled and held on end yokes 33a and 33b. Permanent magnets 35a and 35b are respectively fixed to the inner side surfaces of the side yokes 31a and 31b so as to oppose the center pole 32. In addition, a movable coil (not shown) is arranged to loosely surround the center pole 32. The movable coil can slide along the longitudinal direction of the center pole 32. The voice coil type linear DC motor 30 having the above-described arrangement generates a uniform magnetic flux throughout the stroke of the movable coil. When a current is supplied to the movable coil within the generated magnetic flux, the movable coil is linearly moved forward or backward.
The carriage 21 is fixed on the movable coil (not shown). The carriage 21 holds the optical card 20.
Support members 25a to 25d are respectively arranged on the front and rear portions of two sides of the voice coil type linear DC motor 30. The guide shafts 22a and 22b are fixed on these support members. The carriage 21 is slidably supported by the guide shafts 22a and 22b.
The carriage 21 (optical card 20) can be moved forward or backward by supplying a current to the movable coil (not shown).
The following problems are posed in the conventional carriage driving apparatus described above.
Since the guide shafts 22a and 22b are arranged outside the side yokes 31a and 31b, the pitch of the guide shafts is increased, resulting in an increase in size and weight of the carriage 21. This requires an increase in thrust of the voice coil type DC motor.
Published Unexamined Japanese Patent Application No. 63-181660 discloses a voice coil type linear DC motor as shown in FIG. 2. In this voice coil type linear DC motor, in order to prevent saturation of a magnetic flux near end yokes 33a and 33b and to decrease the weight of the apparatus, side yokes 31a and 31b and a center pole 32 are formed into the shapes shown in FIG. 2.
The following problem, however, is posed when a carriage driving apparatus is constituted by such a voice coil type linear DC motor.
If the carriage driving apparatus is designed such that a carriage is supported on the guide shafts through ball bearings, the ball bearings must be arranged to avoid the side yokes 31a and 31b. In this case, since the space for the ball bearings is very limited, small ball bearings must be used.
The durability of a ball bearing depends on the size. Therefore, the use of small ball bearings leads to a deterioration in reliability, thus posing another problem.